Initially, it is noted that IEEE Standard 802.11-2016 is used as the base reference for disclosures used herein, the entire contents of which are incorporated herein by reference. The IEEE 802.11-2016 Standard is commonly referred to as “Wi-Fi” and is referred to as such herein.
The location of wireless devices can be performed by various methods. These methods may be classified as active, passive and combined active and passive. In an active location scheme, a device that is determining the location or range, the measuring device, transmits certain packets to the device being located, i.e., the target device, and a common method is to measure the time of arrival (TOA) of the response from the target device and compare that to the time of departure (TOD) that the packet was transmitted by the measuring device so as to determine the round trip time, RTT.
In such location systems it is common to use multiple measuring devices to determine the location. In such a scheme, simultaneous TOA and/or TOD measurements are taken by different measuring devices situated at different points, and the location of the target device calculated.
In an active location scheme, TOD may be measured for a packet that is transmitted from the measuring station addressed to the target station. The TOA of the response from the target station at the measuring station is then also measured. If the turnaround time for the target station to receive the packet from the measuring station and to start to transmit the response is known, then the time difference at the measuring station between the TOA and the TOD, minus the turnaround time at the target station will be directly proportional to twice the distance of the target station from the measuring station. For example, if the target station is a wireless device based upon IEEE 802.11 technology, and if the packet transmitted from the measuring station to the target station is a data packet, the response from the target station will normally be an acknowledgement (ACK) packet. If the packet transmitted from the measuring station to the target station is a control packet, for example a ready-to-send (RTS) packet, then the response from the target station will normally be a clear-to-send (CTS) packet. In these two examples, the turnaround time at the target station is defined in the IEEE 802.11 standard as the short interframe spacing (SIFS), which is a preset value. Hence, the time delay, td, between the measuring station and the target station, may be determined from the calculation td=(TOA−TOD SIFS)/2 and the distance between the measuring station and the target station is then c*td, where c is the speed of light. This method of estimating the distance to a target station by measuring the TOD and TOA and accounting for the turnaround time is known in the art.
FIG. 1 is a diagram of a typical location system 100 which includes three measuring stations 110a, 110b and 110c (referred to collectively herein as “measuring stations” or “measuring receivers”). The target station 120 may be a wireless device, such as, for example, an Access Point (AP) that is to be located by the three measuring stations 110. The distance of the target station 120 from measuring station 110a is D1, 130. The distance of the target station 120 from measuring station 110b is D2, 140. The distance of the target station 120 from measuring station 110c is D3, 150. The round trip time, RTT1, determined from the calculation RTT=(TOA−TOD−SIFS), is measured for transmissions from measuring station 110a and this can used to calculate the distance D1 130 using the formula D1=RTT1/2c where c is the speed of light. Similarly, RTT2 and RTT3 measurements result in the determination of distances D2 140 and D3 150. The methods for calculating the location of target station 120 using the distances D1 130, D2 140 and D3 150 are well known in the art.
FIG. 2 is a diagram of an airborne measuring station 110 actively geo-locating target stations 120a, 120b, 120c and 120d. The target station 120a depicts the example of an outdoor device, target station 120b depicts the example of a device inside a residential home, target station 120c depicts the example of a device inside an automobile, and target station 120d depicts the example of an apartment. The obstruction losses for each of the target stations 120a, 120b, 120c and 120d will differ and hence the maximum range at which the airborne measuring station 110 can successfully geo-locate the target stations will differ because the radio frequency (RF) obstruction losses will vary between the target stations. From the airborne measuring station's 110 perspective, the range will be dependent upon two factors: the transmit power of the ranging packet 212, which should be such that the target station will successfully receive it, and the receive sensitivity of the airborne measuring station 110 such that the response packet 224 is received successfully. Transmitting the ranging packet 212 at a sufficiently high power is straightforward, e.g., a higher power amplifier and/or a higher gain antenna, but the receive sensitivity of the airborne measuring station 110 is generally restricted to the noise figure of the receiver.
The signal level, Pr, received at the airborne measuring station 110 is:Pr=Pt+G1+G2−Lfs−Lo  (5)
where Pt=Transmit power from the target station 120;                G1=Antenna gain at the airborne measuring station;        G2=Antenna gain at the target station;        Lfs=Propagation loss, free space; and        Lo=Obstruction loss.        
The obstruction loss, Lo, for the path from the target station 120a and the airborne measuring station 110 can be assumed to be zero as the target station 120a has a line-of-sight path to the airborne measuring station 110. In contrast, the obstruction losses for the other target stations may be in the order of 10 dB for target station 120b, 6 dB for target station 120c and 15-20 dB for target station 120d. There is a problem in that the range of the airborne measuring station 110 to successfully detect the response packets from each of these target stations is limited due to the fixed sensitivity of the measuring receiver of airborne station 110 which is restricted by the noise figure of the receiver and the need to receive a packet without errors.